[llvm] [AMDGPU] SelDAG: fix lowering of undefined workitem intrinsics (PR #126058)

[Matt Arsenault via llvm-commits]

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@@ -8619,6 +8619,11 @@ SDValue SITargetLowering::lowerWorkitemID(SelectionDAG &DAG, SDValue Op,
   if (MaxID == 0)
     return DAG.getConstant(0, SL, MVT::i32);
 
+  // It's undefined behavior if a function marked with the amdgpu-no-*
+  // attributes uses the corresponding intrinsic.
+  if (!Arg)
+    return DAG.getUNDEF(EVT::getIntegerVT(*DAG.getContext(), 32));
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arsenm wrote:

EVT::getIntegerVT(*DAG.getContext(), 32) is an overly complicate way of doing just MVT::i32 as Is done in the 0 case. Plus this should really be using the original output type 

https://github.com/llvm/llvm-project/pull/126058